The present invention relates to an antenna system and in particular but not exclusively to an antenna system for use in a base transceiver station of a wireless telecommunications network.
An area covered by a cellular telecommunications network is divided into a plurality of cells. Each of these cells has a base station arranged to transmit signals to and receive signals from mobile stations located in the cell associated with the respective base station. Mobile stations will be in active communication with the base station associated with the cell in which the mobile station is located.
In order to improve the capacity and/or the signal to interference ratio, beam steering or beam selection techniques have been proposed. With beam steering or selection techniques, a base station is able to generate one or more beams. In the case of beam selection, the base station will be capable of producing a fixed number of beams to cover the cell associated with the base station. In certain conditions, not all of the beams will be selected for communication with a given mobile station. For example, if it can be determined where the mobile station in question lies in the cell, the beam or beams, which cover that part of the cell in which the mobile station is located only will be selected to transmit a signal. Accordingly, that signal for fine mobile station in question Will not be transmitted in the other non-selected beam directions.
Beam steering uses a similar concept. Rather than having a fixed number of beams, the number of beams generated can be varied and the width of those beams can be varied. As with the beam selection technique, one or more beams can be generated, in appropriate circumstances, to transmit a signal over only part of a cell.
With both of these techniques, as not all signals are transmitted throughout the whole cell, the interference generated is reduced. This can allow an increase in capacity to be achieved. Both of these techniques use an array of antennas in order to generate the beams.
A base station allocated to the cell in which a particular mobile station is located, can sometimes receive a plurality of representations of the same signal transmitted from the particular mobile station, the different representation being received at different times. In other words, a signal received by a base station from a particular mobile station could be a new signal or a representation of a previously recovered signal. The various representations of a signal received at different times, is due to multipath propagation.
In dense urban environments the signal generated by the mobile station can reflect off objects such as buildings in the environment. This means that a particular signal may take a number of different routes from the mobile station to reach the base station. These routes are usually of varying lengths so that representations of the same signal arrive at the base station at different times. These representations of signals which arrive at different times are multipath representations of the original signal.
To locate the position of a particular mobile station in a cell, the base station can use the strongest received signal from the mobile station. The base station can then direct the beam of the phased antenna array in the direction of the strongest received signal as it is usually assumed that the direction of the strongest signal strength reception will also be the best transmit direction.
The base transceiver station may determine the direction where the target mobile station may be located by utilising the received signal at the antenna array. A signal transmitted by a mobile station will generally be received by all the antenna elements within the antenna array. There will however, be a difference in the phase of each of the otherwise substantially similar signals received by each of the individual antenna elements. From the relative phase shifts, the base transceiver station determines the direction from which the received signal has been received.
The assumption that the strongest received signal direction is the best transmit direction may not be correct for frequency division duplex systems (such as for Global System for Mobile communications (GSM) systems). In frequency duplex systems, a base station sends signals to a mobile station in a first frequency range and receives signals from the mobile station in a second different frequency range. Different channel conditions may exist for the receive and transmit directions due to the different frequencies. The best transmit direction, especially For mobile stations close to the base station, may be very different to the strongest received signal direction. If the beam is directed to a different location, away from the actual location of the mobile station, the mobile station may not receive the signal from the base station or only weakly receive the signal. To compensate, the base station may then use full power to improve the connection. This increases the interference level in the network, so reducing overall traffic capacity and increasing power consumption within the base station.
Base stations, which have a particular mobile station in close proximity, are susceptible to multipath representations of signals from the mobile station. This makes the mobile stations in close proximity to the base station very difficult to locate because the direction from which the strongest signal is received may be very different to the direction from which the signal was originally transmitted. This means therefore that the overall traffic capacity may be reduced as a large amount of base station resources may be allocated to the particular mobile station.
Additionally, as a beam directed by a phased antenna array is steered towards a target a large angle away from boresight of the antenna array, the beam width increases. This is disadvantageous because the increased beam width away from the boresight can cause unnecessary interference. Conversely, if the beamwidth is too narrow, because of inaccuracies in locating mobile stations, a target mobile station may be missed. Inaccuracies in locating a target mobile station may be caused by, for example, multipath reflections or variations in propagation conditions.
It is therefore the aim of the present invention to provide an antenna system which addresses one or more of the problems as discussed above.
According to a first aspect of the present invention there is provided an antenna system for a transmitter, said antenna system comprising an array of antennas arranged to transmit a signal over part of or all of a coverage area of said transmitter; and control means for controlling the number of antennas which are used to transmit a signal in dependence on the width of the signal to be transmitted.
According to a second aspect of the present invention there is provided a method of transmitting with an array of antennas, said method comprising steps of selecting the number of antenna elements to be used to transmit a signal in dependence on the width of the signal to be transmitted and transmitting a signal over part or all of a coverage area.